U.S. Secretary of State John Kerry walks past American (back) and Israeli flags at Ben Gurion International Airport in Tel Aviv January 6, 2014.BRENDAN SMIALOWSKI/POOL/REUTERS

When White House national security advisor Susan Rice’s security detail cleared her Jerusalem hotel suite for bugs and intruders Tuesday night, they might’ve had in mind a surprise visitor to Vice President Al Gore’s room 16 years ago this week: a spy in an air duct.

According to a senior former U.S. intelligence operative, a Secret Service agent who was enjoying a moment of solitude in Gore’s bathroom before the Veep arrived heard a metallic scraping sound. “The Secret Service had secured [Gore’s] room in advance and they all left except for one agent, who decided to take a long, slow time on the pot,” the operative recalled for Newsweek. “So the room was all quiet, he was just meditating on his toes, and he hears a noise in the vent. And he sees the vent clips being moved from the inside. And then he sees a guy starting to exit the vent into the room.”

Did the agent scramble for his gun? No, the former operative said with a chuckle. “He kind of coughed and the guy went back into the vents.”

To some, the incident stands as an apt metaphor for the behind-closed-doors relations between Israel and America, “frenemies” even in the best of times. The brazen air-duct caper “crossed the line” of acceptable behavior between friendly intelligence services – but because it was done by Israel, it was quickly hushed up by U.S. officials.

Despite strident denials this week by Israeli officials, Israel has been caught carrying out aggressive espionage operations against American targets for decades, according to U.S. intelligence officials and congressional sources. And they still do it. They just don’t get arrested very often.

As Newsweekreported on Tuesday, American counter-intelligence officials told members of the House Judiciary and Foreign Affairs committees at the end of January that Israel’s current espionage activities in America are “unrivaled and unseemly,” going far beyond the activities of other close allies, such as Germany, France, the U.K. and Japan.

“It has been extensive for years,” a former top U.S. security official toldNewsweekWednesday after Israeli Intelligence Affairs Minister Yuval Steinitz, among other top Israeli officials, “unequivocally” denied the Newsweekreport, saying Israel stopped all spying operations in the U.S. after Jonathan Pollard was convicted of spying for Israel in 1987. One anonymous official was quoted in the Israeli media as saying Newsweek’saccount “had the whiff of anti-Semitism in it.”

But a former U.S. intelligence operative intimately familiar with Israeli espionage rejected the anti-Semitism charge. “There is a small community of ex-CIA, FBI and military people who have worked this account who are absolutely cheering on [the Newsweek] story,” he said. “Not one of them is anti-Semitic. In fact, it has nothing to do with anti-Semitism. It has only to do with why [Israel] gets kid-glove treatment when, if it was Japan doing it or India doing it at this level, it would be outrageous.”

Beginning in the mid-1990s, well after Israel promised to stop spying in the U.S. in the wake of the Pollard affair, the FBI regularly felt compelled to summon Israeli diplomats in D.C. for a scolding, two former top counterintelligence officials told Newsweek. During the decade following 9/11, one said, the Israelis were summoned “dozens” of times and told to “cut the shit,” as one, a former top FBI official, put it. But as an “ally,” the Israelis almost always got off with only a warning.

But no matter how stern the FBI’s lecture – usually delivered personally to the embassy’s senior intelligence representative – the Israelis were unmoved, another former top intelligence official said. “You can’t embarrass an Israeli,” he said. “It’s just impossible to embarrass them. You catch them red-handed, and they shrug and say, ‘Okay now, anything else?’”

Always lurking, former intelligence officials say, was the powerful “Israeli lobby,” the network of Israel’s friends in Congress, industry and successive administrations, Republican and Democratic, ready to protest any perceived slight on the part of U.S. security officials. A former counterintelligence specialist toldNewsweekhe risked Israel’s wrath merely by providing routine security briefings to American officials, businessmen and scientists heading to Israel for meetings and conferences.

“We had to be very careful how we warned American officials,” he said. “We regularly got calls from members of Congress outraged by security warnings about going to Israel. And they had our budget. When … the director of the CIA gets a call from an outraged congressman–’What are these security briefings you’re giving? What are these high-level threat warnings about travel to Tel Aviv you’re giving? This is outrageous’ – he has to pay close attention. There was always this political delicacy that you had to be aware of.”

The annual exercise in which the State Department publishes security profiles on foreign countries gave the intelligence agencies huge headaches, he added. “When we were doing the annual threat rating for the U.S. Embassy and consulates [in Israel], it was always a huge debate,” he said. “The intelligence community would always be urging the highest level of threats, while the State Department would be saying, ‘This is not going to go over very well, we can’t give this kind of rating, because there will be certain consequences in terms of travel warnings and restrictions.’ It was always a big, big debate on how you rate the threat over there.”

But the danger is real, he and other former U.S. intelligence officials familiar with Israel’s methods say. Israeli agents “go after senior U.S. Navy officers on shore leave in Haifa, after space industry officials, or scientists with intellectual property, anywhere. This has always been a huge concern for the community.”

In the States, Israeli officials and businessmen are forever trying to lure attractive American targets to visit Israel. Representatives of Maf’at, an administrative body that yokes the Israel Defense Ministry to its military industries, give U.S. counterintelligence agencies great concern, one of the former U.S. intelligence officials said. “They were the ones that really caused us a lot of concern. Because they had a plausible reason to attend all these conferences and defense contracting facilities and whatnot. It was a great cover vehicle for industrial espionage,” he said.

“I remember speaking to one U.S. scientist who was at a conference and being worked by a group from [Israel],” the former U.S. intelligence operative continued. “And this scientist, who was savvy enough to recognize what she saw, said it was really unbelievable how the elicitation techniques were being used – the invitations to come over – basically getting the data dump from a fellow scientist. And the naïveté on the part of the American scientists was really striking. We saw this all the time.”

Israeli officials were brazen enough to pitch even him. After giving a speech at a recent security industry gathering in Washington, he said, he was approached by the commercial attaché of the Israeli Embassy. “He said, ‘Oh, it was great to hear your background, that was a great talk you gave, how interesting,’ and so forth. And I thought, Here it comes, here comes the pitch. And sure enough, he said, ‘Have you ever thought of coming over? We’d love to have you come over, we’ll pay all your expenses while you’re over there, we’ll give you the tour…’ I thought to myself, Come on guys, come on.”

“Their goal,” he continued, “is to get contacts to come out of the U.S. and over there and then wine them, dine them, assess them, see what their weaknesses are. I mean, we had government officials going over there who were offered drugs, like, ‘Hey, do you want to go get some pot?’ What? These are U.S. government officials. The drugs, women coming to your hotel room – they throw everything at you. No matter how high the official.”

On Wednesday, Israeli Intelligence Minister Yuval Steinitz batted away such espionage allegations, saying “Israel does not spy in the U.S., does not enlist spies in the U.S., and does not do intelligence gathering in the U.S.” Likewise, Israeli Foreign Minister Avigdor Liberman said he “would not agree to any spying on the United States, neither directly nor indirectly.” He called the allegations, attributed by Newsweekto intelligence officials who have briefed Congress, “malicious.”

But current and former U.S. intelligence officials stood their ground.

“It really spans the gamut of everything you can think of,” said a former U.S. intelligence official who has been a familiar face in the executive suites of several U.S. security agencies over the decades. “It used to be when French students came to the United States as interns, summer employees and things like that, they all had a French DGSE officer they had to report back to at the embassy,” he said. “Similar things occur with respect to the Israelis … [who] have a lot of Israeli travelers in the United States.”

Such blanket accusations infuriate defenders of Israel, who detect that “whiff of anti-Semitism” in them. Current and former U.S. intelligence officials who opposed Pollard’s early release were also accused of anti-Semitism.

The high number of young Israelis who overstay their visits to the U.S. has been a sticking point in Israel’s drive to get off the U.S. visa-required list. Another is its failure to regularly report lost and stolen passports to Interpol. A bigger issue has been its rough treatment of Arab Americans and pro-Palestinian activists travelling to Israel. But Israeli efforts to pursue U.S. military, scientific and industrial secrets has also emerged as a major hurdle, if not the major hurdle, in normalizing visa relations, according to congressional sources.

“I was in this briefing — there were several” on Israeli espionage by U.S. security officials in 2013, a former congressional aide told Newsweek. “The one I was in had senior staffers from foreign affairs, the full committee, the subcommittee … from judiciary, Republicans and Democrats, senior leadership staff. I don’t think there was anyone in there who didn’t work for a member that wasn’t ardently and publicly pro-Israel,” he said.

“And afterwards, we were saying, ‘No way. You’ve got to be fucking kidding.’” The evidence of Israeli spying was overwhelming, he said. Visa waivers was off the table.

“The voices in the room,” the aide recalled, were, “‘There’s just no way that this is possible.’”